Microstructure and extrinsic magnetic properties of anisotropic Sm(Fe,Ti,V)12-based sintered magnets

Anisotropic SmFe12-based sintered magnets require an optimum microstructure and alloy composition to have a large coercivity mu 0Hc and remanent magnetization mu 0Mr. In this study, we investigated the microstructure transformation and extrinsic performance change in the anisotropic Sm(Fe,Ti,V)(12)-based sintered magnets via reducing the Ti content and partially substituting Fe with Co. Reduction of Ti con-tent in anisotropic sintered Sm8Fe73.5 +xTi8-xV8Ga0.5A(12) magnet eliminated the secondary Fe2(Ti,V) phase, resulting in a remanence of mu 0Mr = 0.8 T and energy density of (BH)max = 113 kJ/m3 (14.2 MGOe) in x = 3. A relatively high coercivity mu 0Hc of 0.6 T is maintained owing to the formation of thin intergranu-lar phase (IGP). Substitution of Co for Fe in the Sm8 (Fe0.95Co0.05)73.5Ti8V8Ga0.5Al2 sintered magnet resulted in a low remanence and low coercivity mu 0Hc. The electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) observations indicate the formation of twins inside 1:12 grains which deteri-orate the texture and remanent magnetization. Magneto-optical Kerr effect (MOKE) microscopy and mi-cromagnetic simulations revealed that although the magnetization reversal starts at the twinned grains, Sm-rich intergranular phase hinders propagation of reversed domains to the neighboring grains, prevent-ing the deterioration of coercivity. However, the formation of ferromagnetic SmFe2-based phase of the MgCu2 type in Co-containing magnet is responsible for a further decrease of coercivity to mu 0Hc = 0.55 T. This study provides an insight on the optimum microstructure to realize high-performance anisotropic bulk SmFe12-based sintered magnets, i.e., ones without twins and ferromagnetic SmFe2-based phase of the MgCu2 type, but with refined grain size, and thin non/weak ferromagnetic IGP isolating the SmFe12 grains. For SmFe12-type phase, reducing the nonmagnetic elements and increasing its saturation magne-tization remain crucial to realize the potential to compete with the Nd2Fe14B phase.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.